fix: improve partial evaluation

[Ocean-OS]

This further improves the partial evaluation implemented in #15494. It accounts for sources that are declared as $state, but never reassigned to (and can be treated as constants), and other (somewhat more complex) cases.

A few possible end goals for this:

• Respect TypeScript type annotations/assertions
• Primitive function type inference/evaluation?:

function get_pi() {
    return 3.14159265358979; //constant value
}

• In call expressions, infer types if the callee is a known global:

String(stuff); //string type
Math.random(); //number

closes #14280

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 0ef68b9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15781-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15781

[Rich-Harris]

This is great, thank you! Hope you don't mind me jumping in, I started out with some minor comments but got carried away. Am keen to get this in to bring us slightly closer to #15292 (comment) — was there anything else you had planned for this PR or is it ready to go in its current form?

[Ocean-OS]

I was thinking of maybe looking into some of the goals I outlined, such as adding more globals and maybe some function analysis, how does that sound?

[Rich-Harris]

Adding more globals sounds good. I feel like function analysis could potentially become a bit of a rabbit hole with diminishing returns; maybe if you have something in mind it could be a good follow-up PR though?

[Ocean-OS]

I started working on function analysis last night, I'll try pushing it (along with more globals and a couple of other smallish things) now and if there are too many possible issues, I'll remove it (and maybe work on it in a follow-up PR)

[Ocean-OS]

That's not too bad, I honestly expected more failing tests.
It appears that some of the tests failed just because the evaluation worked (and they were snapshots that didn't expect the change), but there are definitely some actually failing tests.

[Ocean-OS]

After looking more into the failing tests, I've decided against trying to work on this in this PR, but since it does seem rather promising, I'll work on it in another PR, probably after #15792 gets merged.

[Ocean-OS]

Currently the only failing test has to do with sourcemaps, and I don't quite understand it, could you please fix it?

[Rich-Harris]

looking into it